Combined flow, which refers to a combination of unidirectional and wave-induced oscillatory flows, is omnipresent in natural environments and generates a range of bedforms on sandy beds. Combined-flow bedforms are abundant in coastal and lacustrine environments, as well as in their ancient counterparts in the rock record. However, few experimental studies have focused on the relationship between the morphology and formative hydraulic conditions of combined-flow bedforms. There is thus a wide range of unexplored stability conditions for such bedforms, especially with intermediate oscillation periods. The aim of this research is to undertake new experimental work in the Large Oscillatory Water-Sediment Tunnel (LOWST) in order to address this gap in knowledge. The LOWST has a test section 12.5 m long, 0.8 m wide and 1.2 m high, with the oscillatory motion generated by three pistons. Unidirectional currents were superimposed on these water oscillations using two centrifugal pumps. Fifty-five experiments were conducted in the LOWST, both with and without an initially flattened bed. Bedform development in a 0.25 mm diameter sand bed was studied under pure oscillatory and combined flow conditions with oscillation periods of 4, 5 and 6 s. The maximum orbital velocity (Uo) was varied from 0.10 to 0.70 m/s while the unidirectional component (Uu) was varied from 0 to 0.50 m/s.
This thesis presents new experimental data on bedform initiation and development under unidirectional, oscillatory and, more extensively, under combined flows. In particular, this study was able to populate zones of the Terra incognita region previously defined in the literature (Southard, 1991). In this thesis, the stable bedform configurations under a diverse range of flow conditions were studied and divided into no motion (NM), 2D symmetric ripples (SR), 3D symmetric ripples (SR), 3D symmetric dunes (SR), 3D asymmetric ripples (AR), 3D quasi-asymmetric ripples (QAR), 3D asymmetric dunes (AD), 3D current ripples (CR), 3D current dunes (CD) and upper-stage plane bed (USPB). Each of these bedform stages was described, characterized and reproduced in dimensional and dimensionless phase diagrams. A complete re-evaluation of the nomenclature for combined flow bedforms is proposed, which includes their planform and the cross-sectional geometries in order to better represent the bed morphologies. This new nomenclature was carefully selected in order to integrate the bedform studies both in the unidirectional and oscillatory literature. One of the main changes that allows the integration with the nomenclature used in unidirectional flows is the reclassification of large ripples as dunes. Furthermore, the introduction of the planform and cross-sectional geometries as properties to classify bedforms leads to the definition of a stable phase space for two-dimensional symmetrical ripples and three-dimensional quasi-asymmetrical ripples. Furthermore, the experimental data collected under unidirectional flows larger than 0.30 m/s allows expansion of the current understanding on the bed configuration within the Terra Incognita zone (Southard, 1991), where the phase boundary between combined flow bedforms and current ripples was uncertain. Based on dimensionless analysis, the oscillatory and unidirectional mobility numbers were used to represent the dimensionless phase diagram under combined flows. This set of dimensionless numbers provides a better representation than previous studies that use a friction factor to compute the Shields number. In addition, a quantitative analysis of the bedform cross-sectional geometries has allowed development of new bedform shape predictors based on the formative flow and sediment transport conditions. Moreover, based on the bedform initiation and development experiments, it was concluded that the genesis and growth processes are unique for all types of flows. This result was reflected in the same geometric pattern and development-path of bedforms regardless of the flow conditions. Furthermore, the development-path or bedform growth exhibit the same general trend for different bedform size (e.g., ripples vs dunes), bedform shape (e.g., symmetric or rounded), bedform planform geometry (e.g., 2D vs 3D) and sediment grain sizes. The development of the bed defects during the genesis of bedforms shows a strong relationship with the direction and magnitude of the shear stress throughout the oscillation. In conditions with a symmetric shear stress, the defects grew and propagated symmetrically, whereas when the shear stress was asymmetric, the defects grew and propagated with a predominant downstream direction. Furthermore, for the case of current-dominated combined flow, the maximum upstream shear stress was not large enough to entrain sediment in the upstream direction, resulting in solely downstream migration transport. Bedform development was divided and characterized into four main stages : (1) incipient bedforms, (2) growing bedforms, (3) stabilizing bedforms, and (4) fully-developed bedforms, consistent with the scheme proposed by Baas (1994,1999) for pure unidirectional flows. Finally, a probabilistic model based on the cross-sectional bedform geometries is proposed in order to differentiate between unidirectional, oscillatory and combined flows from their preserved strata. This probabilistic model provides a significant improvement on the present tools to differentiate bedforms in the modern and ancient record.